feat(cli): add update command to self-upgrade from GitHub releases

[Ehco1996]

Summary

• New `ehco update` subcommand that pulls the latest release from GitHub, replaces the running binary in place, and restarts the systemd unit.
• Uses `rename(2)` over the running ELF (safe on linux: kernel keeps the old inode mmapped for the running service while new invocations resolve to the new file), so the swap is atomic.
• Flags: `--force` reinstalls the same version, `--no-restart` skips `systemctl restart` for ops who want to schedule the bounce themselves.
• Linux-only by design (matches `ehco_linux_` assets produced by goreleaser; surfaces a clear error on other OSes).

How to Verify

• `make build && ./dist/ehco update --help` — flags render.
• `./dist/ehco update` on the same version → "already up to date".
• `./dist/ehco update --force` on darwin → fails fast with the OS guard message (full download path only runs on linux).
• On a linux box with the systemd unit installed: `sudo ehco update` should fetch, swap the binary, and `systemctl restart ehco.service`.

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